(Letting the truth get in the way of a good story…)
This line comes from a popular apocryphal anecdote, first aired by Franz Xaver Niemetschek (among other things the first person to write a full biography of Mozart) in 1798:
JOSEPH II: Too beautiful for our ears, dear Mozart, and monstrous many notes!
MOZART: Exactly as many as are necessary, Your Majesty.
In brief, while this fits the posthumous myth of M as free artist on the outside of the Viennese establishment, ruled only by innate genius, it's historically implausible. It doesn’t fit Mozart’s anxiousness to please the Viennese, or with the musical concessions and cuts that he made or sanctioned in the opera.
But the anecdote thrived on "the inability of many of the initial listeners to grasp the music’s psychological involvement in nearly everything that matters in the drama." The ‘true subject’ of the conversation – if in fact it occurred in any form – might well have been M’s accompaniments, which were perceived as "overwrought, distracting and difficult to absorb".
(Source: Thomas Bauman in the Cambridge Opera Handbook for The Abduction from the Seraglio)
Dunno. Is that as in WA Mozart airport, Mozat ties and accessories, the Mozart chocolate, Mozart the fragrance (for men), Mozartplatz in downtown Salzburg, the Mozart Effect or Mozart's Coffee Roasters?
Definitely "anxiousness", i.e. the noun formed from "anxious" in its second sense of wanting something very much (as opposed to "anxiety", which refers to "anxious" in its first sense of worry and nervousness).
I'd also add this non-technical observation: I regard anxiousness more as a motivating force, and anxiety as a feeling or state.